Parameter Tree:
EARTH SCIENCE > LAND SURFACE > FROZEN GROUND > ACTIVE LAYER

Definition:
Active layer, also called frost zone or mollisol is that part of the soil
included with the suprapermafrost layer (i.e., existing above permafrost) that
usually freezes in winter and thaws in summer.


Reference:
American Meteorological Society Glossary of Meteorology.
http://amsglossary.allenpress.com/glossary